1. Field of the Invention
The invention relates to the automated inspection of articles and in particular to optical inspection of transparent or translucent articles at high speeds.
2. Background Information
Production and use of transparent or translucent articles made of plastic or glass such as bottles, jars and vials invariably require optical inspection. This inspection must take place at the high speeds found on production lines.
In order to perform the inspection properly, it must be performed on the most sensitive zones of the article around its entire perimeter. This is normally achieved by inspecting the article while turning it at least one full turn. A known method of performing an overall check of the article at high speeds comprises reading a photosensitive screen illuminated by a light beam which scans the article in successive diametral planes during rotation of the article.
It is advantageous to use a laser beam to scan the article because the intensity and collimation of the laser beam eliminate the need for focusing elements used with other scanning beams. Advantageously, the laser beam should be directed at the side of the article from sufficient distance that it can scan the entire height of the article in contiguous narrow sections at incident angles close to perpendicular.
One type of device known in the art uses a laser beam emitter unit in which a laser beam is directed at a mirror rotating about a horizontal axis to produce a scanning beam and the scanning beam is deflected by a battery of stationary mirrors onto the article at approximately perpendicular angles of incident. Advantageously, the article has axial symmetry. While the article is being scanned by the beam, it is rotated about its own axis for at least one full rotation so that it may be examined completely. This type of machine is described in French Pat. No. A-2 235 365 and U.S. Pat. Nos. 3,942,001 and 3,987,301.
The rotating mirror usually comprises a wheel having a plurality of mirrored facets. For example, the wheel can be about 15 cm in diameter and have 36 facets, each of which will deflect the beam over an angle of about 20.degree., in the vertical plane.
Preferably, the article is transported through the process via a star wheel which is a wheel with a plurality of recesses or U-shaped mountings about its circumference for holding the article to be scanned. At the checking station where the article is inspected, a pair of rollers are used to rotate the article. To provide a clear path for the scanning beam, it is highly useful to place the emitter unit at the center of the star wheel.
To be able to check large volume articles over their entire height, for example article about 40 cm tall, the rotating mirror should be placed at an optical distance of over one meter from the article to be checked. At the same time, the battery of stationary mirrors must be relatively close to the light source so that the dimensions of the apparatus are not prohibitive.
A further problem with the apparatus is in the inertial forces produced in high speed operation. To reduce the need to contend with inertial forces it is preferred that the scan be performed on the fly without stopping the rotation of the star wheel. In order to hold the article against the star wheel during the continuous motion a device such as a belt could be placed in frictional contact with the radially outermost portion of the article to be scanned. This device however has not been used in prior art scanners because it produces a blind spot in the scanning of the article by blocking the path of the scanning beam in a portion of the field.